1. Field
The invention is concerned with the manufacture of an anisotropic layer of cross-linked liquid crystalline monomers (LCP) in contact with an orientating layer on a single substrate, and with optical components having a layered structure comprising an orientating layer, a LCP layer, and at least one additional orientating layer over the LCP layer on a single substrate and their preferred use.
2. Description
Anisotropic transparent or colored cross-linked polymer layers with three-dimensional orientation of the optical axis, either uniform or preset at individual places, are of great interest in many sectors of display technology, integrated optics, etc.
For some years, substances having this property have become known, namely certain cross-linkable liquid crystalline diacrylates and diepoxides. These substances as monomers, that is before cross-linking, can be orientated in the liquid crystalline phase in sandwich cells consisting of, for example, glass plates having an interposed monomer layer with the aid of conventional orientating layers on the two glass plate surfaces or under the influence of external fields, such as strong magnetic or electric fields. In a second phase, the monomer layer can be photo-cross-linked in the cells such that the wall forces acting on the two sides of the monomer layer, or the applied fields, preserve the preset orientation during the cross-linking process.
These external mechanical, electrical or magnetic forces prevent thermodynamic orientation relaxation inherent in liquid crystals and counteract the de-orientating forces of conventional cross-linking processes. In the absence of these external forces a de-orientation or a re-orientation of the liquid crystals usually takes place. The re-orientation from planar to perpendicular at the interface to the atmosphere opposite the substrate surface has been shown in the case of single substrates, see Hikmet and de Witz, J. Appl. Phy. 70:1265-1269 (1991). (Throughout the specification, documents have been identified. The contents of each of these documents are herein incorporated by reference).
Layer structures of liquid crystalline polymers are known, see EP-A-331 233. They are manufactured by orientating a monomer layer in a cell with a voltage applied to the cell plates and then irradiating in a partial region through a mask. By so doing, cross-linking is initiated in the irradiated region only. Subsequently, the direction of the external field is changed and the monomer regions which have not yet been cross-linked are re-orientated with respect to the new field direction. Thereupon, the latter region is also illuminated and thus cross-linked. Clearly, this method cannot yield an orientating structure with high local resolution, since the radical cross-linking reaction, owing to the shading of the mask, does not have sharp boundaries. Further, this method is invariably limited to the use of sandwich cells for orientating the layer structure in an electric field.
Recently, there have become known methods which permit the production of orientating layers with locally variable orientating properties. The orientation of dichroic dye molecules incorporated in the polymer with the aid of photolithographic methods is described in U.S. Pat. No. 4,974,941, the contents of which are herein incorporated by reference.
The orientability and photo-structurability of a liquid crystalline monomer layer in a sandwich cell, the two surfaces of which have been photo-orientated by the laser orientation process described in U.S. Pat. No. 4,974,941, has also recently become known. This process is also limited to orientation of the monomer layer in a cell. The orientation impressed by the cell surfaces is frozen by subsequent conventional photopolymerization of the liquid crystalline monomer layer in the cell. In order to obtain a coated single substrate, the cell must be dismantled after the polymerization (P. J. Shannon, W. M. Gibbons, S. T. Sun, Nature, 368:532 (1994)).
The production of optical strongly anisotropic layers consisting of orientated liquid crystal polymers in cells is also known from Research Disclosure No. 337, May 1992, Emsworth, GB, pages 410-411. There, the manufacture of such layers by placing the liquid crystal monomer in the cell, orientating by means of the two cell walls via rubbed polyimide surfaces of the cell and subsequent conventional photopolymerization in the cell is described. Further, it is mentioned that one of the two glass plates can be removed after the polymerization step in order to thereby obtain a single glass substrate coated with LC polymer. This orientated substrate can, moreover, be provided with a polyimide layer having a new direction of orientation (by rubbing).
After again assembling the thus-prepared polymer substrate in a second orientated sandwich cell, filling this cell with a further monomer layer and subsequent conventional photopolymerization, the optical pitch differences of the two differently oriented LC polymer layers in the cell are added or subtracted.
Since the rubbing of the polyimide layers on the cell surfaces is a macroscopic process, no orientation pattern can be produced with this process, the cells being uniformly orientated over the entire surface. Further, it is extremely time consuming and expensive for the manufacturer of cells having precise plate separations for the realization of uniform optical pitch differences (in the ten nanometer range).
Where, optical retarder layers are required on a single substrate, the manufacture requires, as described in Shannon et al., dismantling the cell. In so doing, the retarder layer must not be damaged. This complicates the manufacturing process rendering it impractical, especially in the case of large substrate areas as are required for high-information computers and TV-LCDs.
Layer structures comprising a film of cross-linked liquid crystalline monomers in contact with an orientating layer of a photo-orientable polymer network (PPN) are described in European Application No. 0 611 981, published Aug. 24, 1994. The manufacture of these layer structures is effected by planar orientation of the liquid crystalline monomers by interaction with the PPN layer and fixing the orientation in a subsequent cross-linking step. Cross-linked liquid crystalline monomers are also referred to as LCPs (liquid crystal polymers) in the following text.
It has now surprisingly been found that liquid crystalline monomer layers can also be applied to and cross-linked on single substrate surfaces which already contain a LCP layer. For this purpose, neither a further orientating counter-substrate of a sandwich cell is required nor are magnetic fields or electric fields necessary for the orientation. This contrasts with EP-A-0 397 263, in which magnetic field orientation of dichroic dyes in a single LC monomer layer for the manufacture of a polarizing film is indicated as being preferred and is actually the sole exemplified process (field-free orientation is, indeed, claimed, but not demonstrated).
Furthermore, it has surprisingly been found that the orientation of these monomer layers on a single substrate is not influenced or destroyed by subsequent polymerization or photo-cross linking. Thus, it is for the first time possible to manufacture on single LCP-orientated substrate surfaces in a simple sequential manner solid films consisting of several orientated liquid crystalline polymer layers. Further, additional layers having different optical and/or electrical functions can be integrated in these complex hybrid layers. This offers for the first time the possibility of realizing not only known but also novel optical components such as polarization-interference filters, optical retarders, polarizers, etc. on single substrates by means of LCPs and to combine and to integrate these components in hybrid layers. Further, additional functional layers such as orientating layers for liquid crystals can be integrated in the hybrid layers.
The present invention provides and opens up new possibilities for optical and electro-optical components and devices using layer structures of the aforementioned kind.
The invention provides a process for making an isotropic layer of cross-linked liquid crystalline monomers in contact with an orientating layer on a single substrate. This process comprises applying an orientating layer onto a single substrate, then applying a layer of a non-cross-linked liquid crystalline monomer, and subsequently cross-linking the monomer. Also provided is an optical component having a layer structure. The component comprises a substrate, a first orientating layer, a liquid crystalline monomer layer, and a second orientating layer. The first and second orientating layers are located on opposite sides of the crystalline monomer layer. At least one of the orientating layers includes a photo-orientating polymer network.
The manufacture in accordance with the invention of an anisotropic layer of cross-linked liquid crystalline monomers (LCP) in contact with an orientating layer comprises applying an orientating layer on a single substrate and applying to this a layer of a non-cross- linked liquid crystalline monomer and subsequently cross-linking the monomer. For the manufacture of more complex layer structures, additional orientating and liquid crystal layers can be applied in further steps and these layers can be cross-linked. Moreover, if desired, optically isotropic de-coupling layers or electrically conducting layers can be inserted or applied between individual LCP layers under the following orientating layers.
The optical component according to the invention is characterized in that at least one of the orientating layers is a layer of a photo-orientating polymer network (PPN) or has a local varying orientating pattern.
Preferably, use is made of monomer mixtures which have nematic, cholesteric, ferroelectric or non-linear optical (NLO) activity at room temperature.
The second and other LCP layers can also be applied directly, i.e. without intermediate PPN layers, to the first.LCP layer and subsequently cross-linked. Thereby, the monomers in the second and subsequent layers take over the preferred orientation of the first or respective underlying LCP layers.
It will be appreciated that the PPN and LCP layers need not cover the entire surface of the substrate, but can cover all or part of the surface in individual and varying manner.
These multi-layer structures are used in optical and electro-optical devices, particularly in the manufacture of liquid crystal cells in which the various LCP layers serve different optical and orientating purposes. They are also used in integrated optical devices, e.g. in strip waveguides, Mach-Zender interferometers and frequency-doubling waveguide arrangements. Finally, these layer structures can be used as a safeguard against counterfeiting and copying.